Antennas for wireless communication equipment, for example pagers, cell phones and WLAN access points must be small in size, light in weight, compact in physical volume, and cheap to manufacture. Flush mounted or built-in internal antennas are therefore often desired or even required. Also, devices that communicate with wireless services often must operate in different frequency bands, due to different geographical band allocation schemes, different wireless providers, different wireless services, or different wireless communication protocols. Such devices accordingly require an antenna or multiple antennas that are responsive to multiple frequency bands. A single antenna is preferable for obvious reasons of size, appearance, and cost. One current example of a single antenna application is multi-band reception and transmission by high-end WLAN access points, which need to accommodate all of the 802.11 a/b/g protocols.
There are already several designs for external multi-band antennas, but a compact multi-band antenna that can be housed internally or on the external device housing is often highly preferred. Unfortunately, existing internal antennas are either not very compact or else trade off performance quality to achieve smaller size. Some antenna designs today also trade off increased cost to reduce size, through the use of materials with high dielectric constants which are usually expensive. One technique used for this is to employ a slow-wave structure to miniaturize the antenna, such as a meander line shape. Unfortunately, that adds to the electromagnetic energy loss incurred. This is inefficient in many applications, and is often a sever disadvantage in applications where battery capacity is a concern.
Various attempts have been made to improve antennas to address the above concerns. One common approach today is to use a patch type antenna.
The classic patch antenna is a rectangular metallic film mounted above a ground plane. However, a patch antenna must be about a half wavelength in size, which for most terminal applications is not suitable. One popular method to reduce size is to use dielectrics with a high dielectric constant. This adds weight and loss and reduces the antenna bandwidth. Another way to reduce size is to incorporate specialized grounding. By doing this, the added inductance to the capacitive planar antenna shifts antenna resonance to a lower frequency. Known as Planar Inverted F Antennas (PIFA), the design of this group of antennas normally includes some kind of slot, thus adding electrical length to the antenna. However the main common characteristics of the standard and shorted patch antennas is that the metal structure parallel to the ground is the main radiating structure, and not the feed or shorting circuits. For monopoles, it is the other way around. Even when monopole antennas use some top-loaded elements, these are reactive elements, not the main radiating structures.
A discussion of some dual- and wide-band examples is provided in GUO et al., “A Quarter-Wave U-Shaped Patch Antenna With Two Unequal Arms For Wideband And Dual-Frequency Operation,” IEEE Transactions On Antennas And Propagation, Vol. 50, No. 8, August 2002. Due to the antenna shape, and also being a patch type antenna, it has not the proper performance and bandwidth.
U.S. Pat. No. 6,788,257 by Fang, et al. teaches a variation of the PIFA-patch type antenna, wherein a driven element is electrically connected to a ground plane with a shorting pin and excites a parasitic shorted radiating patch to produce another resonance mode by the coupling of energy. However, the performance is not adequate for many applications.
Published pat. app. WO 2004/109857 by Iguchi et al. teaches a PIFA-type structure based on parasitically coupling between the directly fed radiating element and the shorted radiating element, but one that has not been able to provide a reasonable bandwidth for the proper performance.
Published pat. app. US 2004/0227675 by Harano and U.S. Pat. No. 4,907,006 by Nishikawa, et al. use parasitic coupling. However, due to non-optimal shapes the overall antennas sizes are big. Published pat. app. WO 03/077360 by Anderson teaches yet other variations, which has a high SAR issue, as it is not completely on one side of the ground plane.
Published pat. app. US 2001/0048391 by Annamaa et al. teaches a variation of the PIFA-type structure that is fed parasitically, e.g. through a conductive strip placed on the same insulating board. The feed conductor of the whole antenna structure then is in galvanic contact with the feed element. However, this technique has not been able to overcome the bandwidth issue due to its patch-type nature. To lower the resonance frequencies, it adds slots or spiral type configurations to increase the efficient path the current flows through.
Of course, other types of antenna structures are possible. For example, published pat. app. US 2004/0150567 by Yuanzhu teaches an antenna using meandering portions and capacitive conductor portions provided on a surface of a dielectric substrate perpendicularly provided with respect to a grounding conductor plate. As noted in passing above, however, this approach is not as efficient as desired due to narrow bandwidth and also increasing loss.
Still another type of antenna structure is represented by published pat. app. US 2004/0061652 by Ishihara et al. This is titled “Top-Loading Monopole Antenna Apparatus With Short-Circuit Conductor Connected Between Top-Loading Electrode And Grounding Conductor” and seemingly contradicts the widely held belief that monopole-type antennas, can operate efficiently over only a narrow band of frequencies. As will be seen in the following discussion, this makes the Ishihara invention particularly relevant to the present invention. However, due to its non-optimum shape and its configuration of the main and parasitic top loading elements, reasonable bandwidth can not be obtained, requiring the use of discrete reactive elements in many cases, as has been indicated in the patent.